Method for continuously measuring the CO2 content in breathing gases

ABSTRACT

A method of continuously measuring the CO 2  content in breathing gases during inspiration and exhalation phases of breathing using a luminous source to pass infrared light rays through an interference filter accorded to CO 2  and a conduit of the gas to a photodetector, comprises passing the breathing gas developed during the breathing phases through the conduit to influence the photodetector to transmit a measured value signal in accordance with the CO 2  content, directing the signal to a breathing air phase recognition unit, storing the maxima and the minima of the measured value signals in each breathing phase in a maximum storage and a minimum storage, dividing the stored values under the control of the recognition unit and directing the values to a computation unit to indicate the logarithm of the values so as to give an indication of the variations of the CO 2  content. The indicated value is then directed to an indicating unit to show the value of the CO 2 .

FIELD AND BACKGROUND OF THE INVENTION

This invention relates in general to a method for measuring gases and,in particular, to a new and useful method for continuously measuring theCO₂ content in breathing gases.

DESCRIPTION OF THE PRIOR ART

Various methods and devices which operate on the principle of infraredabsorption for continuously measuring the concentration of individualcomponents in gases and vapors are known. They are used, for example,for controlling the processes in chemical plants for the environmentalcontrol of air and also in medicine. The measuring effect of theinfrared absorption devices is based on the specific absorption of theradiation of heteroatomic gases in the infrared spectral region. Theradiation is absorbed at definite frequencies associated with thenatural oscillation of the molecules. With the exception of themonoatomic rare gases and the diatomic element gases, such as oxygen andhydrogen, any gas possesses an absorption spectrum in its infraredregion which is composed of individual absorption bands and is specificfor the respective gas.

In the known devices, the absorption takes place in a cell which isintegrated within the measuring instrument and through which the gassample is conducted. For the intensity I of an undulatory radiationsubjected to absorption and having passed through a medium layer with athickness 1, LambertBeer's law is applicable according to which

    ______________________________________                                        I = I.sub.o . e.sup.-.sup.k.1.c                                                               where I.sub.o is the light intensity                                          at its entrance into the medium                                               k is the extinction coefficient                                               l is the path within the cell                                                 c is the concentration of the gas                             ______________________________________                                    

All of the known gas analysis devices operating on the infraredabsorption principle use the radiation absorption in the infraredspectral region, which is specific for the gas to be measured. Thefollowing operational methods are used:

Two-channel method

The equiphase modulated radiation emitted by two incandescent coilspasses in parallel beams through a reference chamber and a measuringchamber and reaches the receiving chamber. The latter is subdivided intotwo compartments by means of a diaphragm capacitor. Both compartmentsare filled with the gas to be measured. The measuring chamber containsthe gas to be measured while the reference chamber is filled with aninert gas, such as nitrogen, which does not absorb any radiation. Bothbeams of radiation, through the reference chamber, as well as throughthe measuring chamber, are periodically, and in phase, interrupted bymeans of a rotating screening wheel.

As soon as, due to the presence of the gas to be measured, the beamtransmitted through the measuring chamber is partly absorbed, thedifferential signal therby produced causes periodic pressure andtemperature variations in the receiving chamber. These variationsgenerate capacitance variations in the diaphragm capacitor which are afunction of the concentration and may be made perceptible in a gasindicator.

Single-channel method

The radiation which is emitted by an incandescent coil and modulated inopposite phase by a rotating screening wheel passes in parallel througha reference chamber and a measuring chamber which are provided in atwo-part cell, and reaches the receiving chamber. The receiving chamberis subdivided into two compartments and filled with the gas to bemeasured. Both compartments act on a diaphragm capacitor. As soon as,due to the presence of the gas to be measured, the radiation is partlyabsorbed in the measuring chamber, the differential signal therebyproduced causes pressure and temperature variations. These variationsgenerate capacitance variations in the diaphragm capacitor which are afunction of the concentration and may be made visible in a gasindicator.

Single-channel method with a reference receiver

The radiation of a Hg-vapor lamp beamed by means of a quartz condenserpasses through an interference filter to a beam splitter in which onehalf of the light is deviated to a reference receiver. The other half ofthe beam passes through the measuring cell. In a program-controlledmanner, the measuring cell is alternately filled with an inert gascorresponding to the zero point, and the gas sample to be measured. Eachtime, a two-beam photometer compares the two values, the zero point andthe measured value with the value supplied from the reference receiver.While inert gas flows through the measuring cell, a motor-drivenpotentiometer can bring the measuring bridge into balanced state. Thescavenging and balancing intervals are adjustable and are chosenaccording to the contamination of the cell to be expected (periodical"Wasser, Luft and Betrieb" 18, 1974).

Another known infrared-absorption measuring device for measuring the CO₂content in the expiration air operates without a reference gas in thebeam path of the luminous source which, in this case, is a NiCr wirecoil. Two paths of rays are provided in the device, which differ fromeach other by the interposition of a reference filter and an analyticalfilter, respectively. In the order of transmission after the luminoussource, the rays pass the measuring cell, then, according to theswitching position, either the reference filter or the analyticalfilter, and subsequently, a broad-band filter covering the wavelengthsof both the reference filter and the analytical filter, and aphotodetector. The measuring device also comprises further well-knownequipment for amplifying the measuring signals coming from thephotodetector, the synchronizing switching mechanism, etc.

The measuring cell is mounted in a bypass of the expiration air. Thebypass current of the expiration air which is moved by a smalltransistorized pump flow through small passages into the measuring cellto pas through the entire cross-sectional area thereof. Similar passagesare provided at the outlet side. With this measuring cell, the measuringdevice is capable of testing up to forty breathing strokes per minutehaving a rate of flow of 0.6 1/min.

The wave length of the reference filter is approximately 5 μm, that ofthe analytic filter about 4.26 μm. The broadband filter comprising alead-tellurium layer and a glass layer prevents the passage of rayshaving wavelengths shorter than 3.75 μm and longer than 5 μm.

The wavelength of the reference filter as been chosen so as to preventan absorption in the gas sample, thus in the expiration air, even in thepresence of CO₂. The wavelength of the analytic filter, however, largelycorresponds to the absorption band of the substance to be measured. Theresulting difference of the measuring signal in the photodetector is themeasured value.

In practice, the time necessary for each of the measurements is largelydetermined by the intake and exhaustion of the respiration air, alongwith the scavenging of the cell. The scavenging of the cell is aproblem. This is why the narrow passages distributed over the entirecrosssectional area are necessary. A uniform scavenging of the cell,however, can be obtained only if all the passages are clean (D. W. HILLand R. N. STONE, J. SCI. INSTRUM. 1964, Vol. 4J).

The methods mentioned in the foregoing, using the infrared absroptionfor measuring a gas, are disadvantageous for a determination of the CO₂content in breathing gases. The measuring cells with the measuringequipment are too big and heavy to permit a mounting directly in therespiration circuit. In addition, even with the use of the largestcells, the rate of flow of the gas to be measured does not exceed about60 l/h. With such a small sample quantity, they cannot be mounteddirectly in the breathing-gas stream. Consequently, they must besupplied through a bypass. The time lag of the scavenging and refillingof the cell with the breathing gas to be measured caused by the bypassmakes a direct control of the respiration phases almost impossible. Withthe mentioned measuring methods, the variations of pressure andtemperature in the breathing-air stream lead to losses of sensitivity inthe measured value. The taking of a gas sample from the respirationcircuit and its supply through a bypass requires a costlyinstrumentation if a disturbance of the result of measurement by otherinfluencing quantities, like gas flow, elasticity of the lungs, strokevolume, etc., is to be prevented.

Reference gases with the necessary flow direction arrangements make themeasuring devices complicated and they are only an auxiliary means fordetecting or compensating the sensitivity variations and zero-pointdisplacements caused by the combination of the cells and the aging ofcomponent parts, such as emitters, receivers, etc.

SUMMARY OF THE INVENTION

The present invention provides a measuring method for a delay-freedetermination of the continuous CO₂ content of breathing gases at theend of the expiration phase, in which unnoticed measuring errors, forexample, due to a contamination of the cell or aging of component parts,are securely eliminated.

In accordance with the invention, the measured values of a measuringcell which is placed in the breathing-air stream, are processed in asignal processing unit which is controlled by a breathing-air phaserecognition unit, the respective maxima and minima of the measuredvalues of each breathing phase are stored in a maximum-value storage anda minimum-value storage, these values, controlled by the breathing-airphase recognition unit, are divided and the logarithm is taken thereofon a computation unit and, since ##EQU1## the CO₂ content at the end ofthe expiration phase is indicated in an indicating unit.

According to a development of the invention, the continuously measuredvalue of the CO₂ content in each breathing phase, after division by theminimun value from the minimun-value storage and logarithmation in thecomputing unit, is applied to the output terminal. The particularadvantage of this method is that no reference gas and no movablecomponent parts are needed. By placing the measuring cell directly inthe breathing-air stream and, thus, using the entire breathing gasvolume as a sample, a delay-free, accurate and, because it isindependent of additional conditions of a bypass, representative andreproducible measurement is ensured. Further substantial advantages ofthis method are the elimination of the influence of other gas componentsof the breathing gas and the avoidance of the critical problem of a zeropoint.

The gas measuring device for carrying out the gas measuring methodcomprises a replaceable measuring-cell tube which is received in amounting designed as a stable support adapted to be warmed up andaccommodating a luminous source, a system of lenses, an interferencefilter and a photodetector.

The advantages obtained with this embodiment are to be seen in that dueto the possibility of providing small dimensions, the device can easilybe placed in the breathing-air stream close to the mouthpiece, withoutannoying the patient. The measuring-cell tube is easily replaceable and,therefore, complies with the requirement for sterility imperative in themedical art. Upon removal, the tube may either be sterilized or replacedby another. The measuringcell tube is a simple structure comprising nofurther equipment necessary for it function and, consequently, it isinexpensive.

The device for carrying out the gas measuring method further comprises alight modulator which is provided between the luminous source and theinterference filter. This is advantageous in cases wherelight-transmitting connecting tubes are used for avoiding stray lighteffects.

Accordingly, it is an object of the invention to provide an improvedmethod of continuously measuring the CO₂ content in breathing gasesduring the inspiration and exhalation phases of breathing and using aluminous source to pass infrared rays through an interference filterwhich is accorded to CO₂ and through a conduit of the gas to aphotodetector, comprising continuously passing the breathing gasdeveloped during the breathing phases through the conduit to influencethe photodetector to transmit a measured value signal in accordance withthe CO₂ content, directing the signal to a breathing air phaserecognition unit, storing the maxima and the minima of themeasured-value signals in each breathing phase in a maximum storage anda minimun storage, dividing the stored values under the control of therecognition unit and directing the values to a computation unit toindicate the logarithm of the values so as to give an indication of thevariations of CO₂.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference should be had to the accompanying drawings and descriptivematter in which there is illustrated a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of the gas measuring method of the invention;

FIG. 2 is a curve indicating variations of the primary signal during thebreathing phases;

FIG. 3 is a curve showing the variation of the control signal;

FIG. 4 is a diagram showing a control signal as a pulse signal; and

FIG. 5 is a cross-sectional view of a measuring cell constructed inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings in particular, the device for measuring theCO₂ content in a breathing gas, comprises a measuring cell,generallydesignated 1, which includes a tubular measuring member made ofa translucent material which is received in a mounting 17 which may bedisposed close to a patient's mouth and between a mouthpiece for thepatient and a well-known Y piece connection which is provided in therespiration line. The measuring-cell tube 16 is made of a suitableinfrared transmitting plastic and it is intended for a single use. Inorder to prevent condensation of water droplets on the inside ofmeasuringcell tube 16, the tube is heated through an appropriatelydesigned heater 50 which is engaged on the mounting 17 or embeddedtherein in order to effect the heating thereof, such as by electricalresistance heating. Mounting 17 is preferably a concentric tube made ofa resistance material having a positive temperature coefficient. Apositive temperature coefficient material changes its resistance withthe increasing temperature so that at temperatures below the switchingtemperature, the resistance increases only slightly with thetemperature. Upon exceeding the switching temperature, however, thetemperature dependence of the resistance rises by a multiple. Thus, withthe positive temperature coefficient material resistance connected to aconstant voltage source, the temperature is kept at a constant level ina reliable and simple manner.

In the preferred form, the mounting 17 comprises a tubular member whichmaybe heated up to approximately 40° C which is provided with openingsor slots through which an infrared radiating light ray may pass. Themounting is provided with a cavity for mounting a luminous source 19providing an infrared radiating source. A light modulator 20 is alignedwith the light source 19 along with an interference filter 18 (about4..25 μm), and a photodetector 23 which is responsive to infrared lightso that the emitted light, prior to its incidence on the photodetector23, must pass through the interference filter 18 and the measuring celltube 16. To direct the light through tube 16 in an optimum manner, therays arebeamed or controlled by means of a lens system which includespreferably several lenses such as the lens 26.

The equipment of the measuring cell 1 is completed by current supplylines 22, a holder 21 for the luminous light source 19, a holder 24 forthe photodetector 23 along with signal lines 25 from the photodetectorto a further processing device for processing the signals, such asindicated inFIG. 1 by the numeral 2.

A variation of the light intensity results in a directly proportionalvariation of the resistance of photodetector 23 which is responsivethereto. In accordance with LambertBeer's law, however, the functionalrelationship between the light intensity and the CO₂ concentration inthemeasuring cell is

    I = I.sub.o . e.sup.-.sup.k.l.c.

Thus by measuring the resistance of the photodetector, an exponentialmeasure of the CO₂ concentration is obtained as the measuringsignal.This may be done, for example, by means of a resistance bridge,but other circuits may also be usable.

As air is inhaled and exhaled in the breathing phases through themeasuringcell tube 16, the measuring signal, after conversion in thesignal processing unit 2, takes a typical shape as indicated at 9 inFIG. 2. Signal processing unit 2 transforms the response of thephotodector 23 to the light intensity variation into a suitably strongprimary signal 28.

During the inspiration phase 11, no CO₂ is present and the lightabsorption is at its minimum 13, as shown in FIG. 2. During theexpirationphase 10, the CO₂ concentration increases continuously toreach its maximum at the end of the expiration. After a short delaycaused by the necessary scavenging of the measuring cell tube, theinflection point of the curve between expiration and inspiration isfollowed by a steep drop of the measuring signal.

This deep drop of the measuring signal is used by the breathing phaserecognition unit 5 to produce a signal for the controlling of themaximum-value storage 3 and a minimum-value storage 4, as shown in FIG.1.The breathing air recognition unit 5 comprises a differentiating unitforming the derivative 29, as shown in FIG. 3, of the primary signalwith respect to the time. By superposing a voltage 14, a clearintersection between the abscissa and the derivative is obtained whichis used for producing a pulse signal 15, as shown in FIG. 4, by means ofa comparator and a following monostable multivibrator.

With the pulse signal 15, maximum-value storage 3 and minimum-valuestorage4 are controlled so that, during a breathing cycle, that is,inspiration and expiration, these storages determine the maximum 12 andthe minimum 13values, respectively, store them and, as soon as theexpiration is terminated, forward them to a computing unit 6. Thecircuitry of suitable maximum-value 3 and minimumvalue 4 storagesbelongs to the prior art.

In computing unit 6, the quotient of the primary signal 9 by the storedvalue of the minimum-value storage 4 is continuously formed in time.Subsequently, the logarithm is taken of the result and this representsthevariation in time of the CO₂ concentration in each breathing phase.

That is, for the gas mixture, the above equation reads: ##EQU2##

Since in the inspiration phase, c_(CO).sbsb.2 is approximately zero andc_(N).sbsb.2O in the inspiration and expiration phases differ onlyinsignificantly from each other, the results are, after division##EQU3##and after logarithmation ##EQU4##

In consequence, the output of the computing unit is ##EQU5##

Therefore, variations of I_(o) caused, for example, by aging ormisadjustment of the optic, cannot lead to a measuring error. For thesamereason, the presence of N₂ O or other gases, the concentration ofwhich changes only to an insignificant extent during the inspiration andexpiration phases, does not affect the measurement either.

To determine the value of the CO₂ concentration at the end of theexpiration phase, in the computing unit 6, the quotient between themaximum 12 and minimum 13 values is alternately formed and the logarithmthereof is subsequently taken.

Due to the physical and physiological interrelationship, the maximumvalue 12 of the CO₂ concentration is identical with the value at the endofthe expiration phase.

It is advisable to provide for an indication only of this value 12corresponding to the end of the expiration phase. This is done in theindicating unit 7b. The CO₂ value variable in time may be supplied toaterminal 7a to which a stenotype machine or an oscillograph may beconnected.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A gas measuring method on the principle ofinfrared absorption for continuous measuring of the CO₂ content inbreathing gases while using a measuring cell along with a luminoussource, an interference filter accorded to CO₂, and a photodetector,wherein the measured values of the measuring cell which is placed in thebreathing-air stream are processed in a signal processing unit which iscontrolled by a breathing-air phase recognition unit, the respectivemaxima and minima of the measured values in each breathing phase arestored in a maximum-value storage and a minimum-value storage, saidvalues, controlled by the breathing-air phase recognition unit, aredivided and the logarithm is taken thereof in a computation unit and,since ##EQU6##the CO₂ content at the end of the expiration phase isindicated in an indicating unit.
 2. A method of continuously measuringCO₂ content in a breathing gas during the inspiration and exhalationphases of breathing and using a luminous source to pass light raysthrough an interference filter accorded to CO₂ and a conduit of the gasto a photodetector, comprising continuously passing the breathing gasdeveloped during the breathing phases through a conduit to influence thephotodetector to transmit a measured value signal in accordance with theCO₂ content, directing the signal to a breathing-air phase recognitionunit, storing the maxima and the minima of the measured value signals ineach breathing phase in a maximum storage and a minimum storage,dividing the stored values under the control of recognition unit anddirecting the values to a computation unit to indicate the logarithm ofthe values so as to give an indication of the variations of the CO₂content in the breathing gases.
 3. A method according to claim 2,including indicating the CO₂ content in each breathing phase in anoutput terminal after division by the minimum value and the minimumvalue storage and logarithmation in the computing unit.